On the Effects of Aliovalent Substitutions in Thermoelectric Zintl Pnictides. Varied Polyanionic Dimensionality and Complex Structural Transformations─The Case of Sr₃ZnP₃ vs Sr₃Al_xZn_1–xP₃

Abstract

The structures and the transport properties of a novel family of Zintl phosphides and arsenides with the formula AE 3 ZnPn 3 and the solid solutions AE 3 Al x Zn 1−x Pn 3 , AE 3 ZnAs y P 1−y (AE = Sr, Eu; Pn = P, As) are reported. Crystals of nine new phases have been obtained via Pb-flux reactions and used for structural work by means of single-crystal X-ray diffraction methods. The derived orthorhombic structure is without a direct analogue and features unusual structural units, where the Zn atoms are in both distorted tetrahedral and trigonalplanar coordination of pnictogens. Electronic structure calculations reveal moderately wide band gaps for Sr 3 ZnP 3 and Sr 3 ZnAs 3 , on the order of 0.70 and 0.63 eV, respectively. Electrical transport measurements above room temperature indicate relatively high resistivity values above 500 K (ρ ≈ 4.8 Ω cm and above), but some of the samples exhibit very high Seebeck coefficients, as large as 300 μV/K at 560 K for Sr 3 ZnAs 3 . Aliovalent substitutions in AE 3 ZnPn 3 , achieved by the partial replacement of Zn 2+ with Al 3+ cations, promote occupational and positional disorders, which cause structural transformation toward the disordered variant of the Sr 5 Al 2 Sb 6 structure type. Such substitutions also change the dimensionality of the polyanionic sublattice in the resulting quaternary AE 3 Al x Zn 1−x Pn 3 phases. Preliminary transport property data on the latter reveal nine times lower electrical resistivity (ρ 500 ≈ 0.5 Ω cm) together with a significantly enhanced Seebeck coefficient, α max ≈ 430 μV/K at 560 K.

Novel ternary Zintl phosphide halides Ba3P5X (X = Cl, Br) with 1D helical phosphorus chains: synthesis, crystal and electronic structure

“…0.73 eV for both compounds, even though it is expected to observe smaller band gaps for compounds with heavier elements within the same family of isostructural phases. 54 On the contrary, the calculated band gap for the Ba 3 P 5 Cl model in space group Cc is significantly larger ( ca. 1.2 eV, Fig.…”

Sn(IV) Polyanionic Materials as Efficient Visible-Light-Driven Water-Splitting Photocatalysts

“…The small cations (i.e., 2 × Na) occupy the small voids, and the large ones (6 × Rb) fill the large voids (see Figure ) to stabilize a charge balance Rb 3 NaSn 3 Se 8 with the P 4/ nbm space group. This type of crystal decoration is reminiscent of the Zintl phases, in which covalently bonded pnictogens are surrounded by cations, resulting in a charge-balanced crystal. , …”

Exploiting the fraternal twin nature of thermoelectrics and topological insulators in Zintl phases as a tool for engineering new efficient thermoelectric generators

“…[36][37][38] This interest led to the rapid expansion of Zintl phases into the realm of transition and rare-earth elements; when such elements of open core-shells are incorporated, the attendant structural diversity and physical properties can reach new heights. 20,30,[39][40][41][42][43][44] The actualization of closed-shell electronic configuration in these materials brings about their characteristic features as semiconductors or insulators as well as the diamagnetic or temperature independent paramagnetic behavior. 26 The Zintl phases combine the features of narrow band gap, intricate atomic bonding and amenability to a high degree of disorder and present a fertile playground to explore the various electronic and transport properties of materials of interests ranging from the topological phases of matter 45,46 to applications in thermoelectrics 47 and photovoltaics.…”

“…36–38 This interest led to the rapid expansion of Zintl phases into the realm of transition and rare-earth elements; when such elements of open core-shells are incorporated, the attendant structural diversity and physical properties can reach new heights. 20,30,39–44…”